Conventionally, in a case where the center frequency of a laser in an external cavity single mode is continuously swept, a center wavelength is set by altering the operation temperature of the laser. However, there is a problem in that oscillation wavelengths become discontinuous due to mode hopping at changes in temperature. In addition, a response time to a change in the center wavelength resulting from the changes in temperature is about a few seconds, which makes it impossible to achieve high speed changes.
Accordingly, as a conventional method of avoiding the mode hopping, there has been proposed a configuration in which an optical filter or an external cavity length is variably set in accordance with the changes in the center wavelength (see, Patent Document 1, for example).